Cracking hydrocarbon oils



sept; 12,1944.

l P. QSTERGAARD GRACKING HYDROCARBON OILS Filed Feb. 3, 19392-Shee.tS-Sheet 1 ONH wm n@ om umd on 0N Sept 12 1944 P. osTERGAARDcnAcxIne HYnRocAnBoN OILS Filed Feb. 5, 1939 2 Sheets-Sheet 2 H ON bb RuGAL.

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NH QQUQQU Patented Sept. l2, 1944 UNITED STATES oRAcxmG mnocannon omsPovl Ostergaard, Mount Lebanon, Pa., assigner to Gulf Oil Corporation,Pittsbnrth, Pa., a corporation o! Pennsylvania Application February s,1939, serial No. 254,481

17 Claims.

. My invention relates to processes for cracking hydrocarbon oils toobtain gasoline hydrocarbons useful as motor fuel and having highanti-knock value when so used. It relates more particularly to improvedmethods of obtaining gasoline hydrocarbons from crude petroleum stocks,wherein such stocks are first distilled in order to recover a pluralityof fractions of different boiling-point ranges, one of these fractionsis segregated and subjected to catalytic cracking, whilev anotherfraction is subjected to thermal cracking, and normally gaseoushydrocarbons thereby produced are recovered and delivered to the thermalcracking operation in order to promote the degree of cracking obtainedtherein and to effect a conversion of said gaseous hydrocarbons togasolinelike hydrocarbons of high anti-knock value; all as more fullyset forth and described hereinbeloW.

Within the past i'ew years, great strides have been made in the art ofcracking petroleum oils. These advances, for the most part, derive fromthe ever-increasing incentive to produce a maximum yield of motor fuelof maximum anti-knock value. Those processes which are believed to havemade substantial progress in this direction may in general be dividedinto two categories; first, processes involving conversion orpolymerization of the normally gaseous hydrocarbons produced inoil-cracking operations, and second, catalytic oil-cracking processes.But While much thought and time has been expended in the development ofthese individual types of processes, comparatively little thought hasbeen given to methods of combining them advantageously in a unitaryconversion operation.

A considerable number of gas-polymerization processes have been proposedand some have been operated on a large scale. The opinion is growing,however, that such processes are likely to nd their greatest utilityeither for the recovery of gasoline-like hydrocarbons from natural gasesor, as in the case of certain catalytic-type polymerization processes,for the manufacture on a relatively small scale of special fuels, suchas isooctane, the market for which is still comparatively limited. In sofar as gases derived from renery cracking operations are concerned, itis believed that the greatest advance in the art has been made in thedevelopment of oil-cracking processes of the so-called gas-reversiontype, in which normally gaseous hydrocarbons produced in cracking thehydrocarbon oil are recirculated to the conversion zone.

A successful and widely adopted process of this character is thatdisclosed in my prior U. S. Patents Nos. 2,135,014, 2,135,108 and2,135,109.

Others prior to me had proposed to recirculate C: and C4 hydrocarbons tothe cracking zone of an ordinary oil-cracking operation, but, at leastin such cases a's involved the return of these gases for reactionpurposes and without the presence of non-reactive or diluent gases, hadconfined themselves to operations conducted under the ordinaryoil-cracking conditions which would have obtained for the same oil. Inmy prior patents referred to above, I disclosed a process of thisgeneral type, in which the degree of conversion per pass and theoperating temperature are increased above those which would normallyobtain and in fact above those which could be maintained under otherwisesimilar conditions and without undue carbon dposition, if the same oilwere cracked in the same apparatus without recirculation of the reactivenormally gaseous hydrocarbons.

My aforesaid process has been successfully applied to a wide variety ofcracking stocks, with highly advantageous results. Strikingly enough,however, it has been found that the optimum advantages of gas-reversiontype of system generally are obtained when the process is applied to thecracking or re-forming of naphthas and similar light stocks containingsubstantial amounts of hydrocarbons within the gasoline boiling-pointrange. On the other hand, my aforesaid process has also beensuccessfully and advantageously applied to the cracking of heavierstocks, such for example as gas oil and reduced crudes.

As distinguished from thermal cracking processes, a large number ofcatalytic cracking processes have been proposed, all characterized bythe presence of a catalyst of one type or another in the conversionzone, and usually by the employment of relatively low pressures. Theso-called Houdry process, employing as a typical catalyst an activatedhydrosilicate of alumina or the like, is perhaps the best known of thesecatalytic processes, and its adoption is being widely advocated. Onaccount of the necessary periodic interruption of this type of process,in order to effect periodic regeneration of the catalyst employed, andfor certain other reasons, the pressures employed in these catalyticprocesses are for the most part extremely low, rarely if ever exceeding50 pounds per square inch inthe catalyst contact zone. For this reason'it is somewhat difcult to combine gas krecirculation with suchcatalytic cracking operations in an vefiicient andA effective manner.

Moreover,- these types of processes in general gaseous hydrocarbons (asis true of low-pressure operations generally), and they are for the mostpart lacking in ultimate economy and advantages with respect to thecracking or re-forming of naphthas and similar light stocks. That isespemediate products produced in catalytic cracking .operations of thistype, it is ordinarily preferable to withdraw such oil as produced fromthe catalytic cracking unit, and to subject it to conversion to gasolinein a. separate thermal cracking unit.

In accordance with my invention, the maximum advantages and economies ofboth types oi' processes described hereinabove are secured together withadvantages and economies which could not be obtained with respect toeither type of operation considered alone.

My invention contemplates the separatiomby distillation, of a crudepetroleum into a plurality of fractions of different boiling-wintranges, including a naphtha fraction suitable for re-forming to highanti-knock gasoline motorfuel, and at least one heavier fraction. I'henaphtha thus recovered is re-formed in a gas-reversion type operation(preferably conducted in'accordance with the disclosures of my priorpatents referred tend to produce rather high yields of normally 3 to 4carbon atoms per molecule, a considerable portion of which are producedin the re-forming operation itself. A heavier fraction recovered fromthe distillation of the original crude is subjected to separatecatalytic cracking, ordinarily at low pressures, and normally gaseoushydrocarbons containing 3 to 4 carbon atoms per molecule produced inthis operation are delivered to the naphtha re-forming operationreferred to hereinabove.

As will be shown below, the heavy fraction thus subjected to catalyticcracking will ordinarily comprise a residual fraction, such as a reducedor topped crude, which may or may not contain virgin gas-oilconstituents. In` some instances, it is desirable, as shown hereinbelow,to recover virgin gas-oil contained in the original crude as a separatefraction, and to subject this fraction to thermal conversion in thepresence of recirculated Ca and C4 hydrocarbons. The products of thiscracking step are conveniently combined with the products from thenaphtha re-i'orming step for ultimate fractionation and recovery. Gasoilrecovered in the catalytic cracking unit may also be cracked in asimilar manner.l Provision is also made for separately cracking gas-oilrecovered as an intermediate condensate in any and all of the thermalconversion units, the products from this' operation being combined withthe other thermal cracking products for fractionation and recovery. A

I have iound it highly advantageous in a system of this character toeilect'the recovery of `assunse recovery of C: and C4 hydrocarbons andafter separation ci' the condensate thereby obtained are passed to anabsorber where the remaining C: and C4 hydrocarbons are picked up in thefresh naphtha or other charging; stock entering the thermal crackingunit and thereby transferred to the conversion zone of the latter.Ordinarily, naphtha is used as the absorbent, and the absorbed C: and C4hydrocarbons enter the naphtha re-i'orming zone. Condensate recoveredbetween the rectifying stage and the absorber is delivered to theother.v thermal conversion zones in proportion to the requirements ofeach.

As will be shown in further detail hereinbelow, I prefer to introducethe gases produced in the catalytic cracking unit into .the thermal icracking unit. at a point prior to the aforesaid step wherein iseffected the preliminary condensation of C: and C4 hydrocarbons: morespecifically I introduce these gases (as well as any gases recovered inthe initial crude distillation) into the rectifying zoneof the thermalcatalytic unit, wherein condensation and removal of stabilized gasolineis eiected. This procedure permits the recovery of any gasolineconstituents contained therein, as well as a portion of the C3 and C4hydrocarbons present, prior to the absorber, and lightens the vloadcarried by the absorber.

\ While various specic modications of the thermal cracking units arepossible, I disclose hereinbelow, as illustrative examples, two generaltypes of thermal cracking umts, namely, the conventional type in whichtar, gas-oil and gasoline are recovered as individual fractions, and thevso-called pressure coking type, wherein coke and gasoline represent theultimate products recovered. In connection with the latter type ofoperation, I have found' it advantageous to introduce heavy residual ortarry bottoms, separated from the heated charging stock entering thecatalytic cracking unit prior to the passage of the separated vaporsthrough the catalyst, into the ooking zone of the thermal cracking unit.By thus delivering the aforesaid bottoms, along with intermediatecondensates from the thermal and catalytic units, to the thermalconversion unit, it is possible to run the entire system to ultimateyields of gasoline, coke and dry gas. the latter being substantiallyfree from C3 and C4 hydrocarbons.

My invention also contemplates such operating details and modifications,and such additional loperative advantages and economies, as willhereinafter be found to obtain.

In order that my invention may be fully set forth and understood, I nowdescribe, with reference to the drawings accompanying and forming partof this specification, various forms and Inanners in which my inventionmay be practiced and embodied. In these drawings,

Figure 1 is a more or less diagrammatic elevational view of apparatusfor manufacturing gasoline of high anti-knock fuel from petroleum oil inaccordance with my invention, and includingapparatus for separatelysubjecting individually selected fractions to catalytic cracking andshow essential details of operation without encumbering this disclosurewith description and explanation of many more or less obvious minornasales modifications, the application of which will be obvious to thoseskilled in the art.

Similar reference numerals designate similarV temperatures are oi theorder oi '100 to '150 FQ 'I'he preheated crude then passes through aline l having a valve 1 into a distilling column or ilash tower l ofmore or less conventional design. Gases and vapors leaving the top ofthe tower I pass through a line 9 to a condenser I II and thence to aseparator I I from which cool reilux oil (naphtha) is returned to thetop of the tower I through a line I! having a pump I4. By reason of theheat imparted to the crude oil and the low or atmospheric pressuremaintained in the column l, distillation takes place, and a considerableportion of the crude oil is vaporized, while condensation andrectiilcation in the column 8 is effected by means of the refluxsupplied through the line I3 or inany other suitable manner. Lightvirgin gasoline removed in the distillation collects in the separator IIand is withdrawn therefrom through aline I5. Heavy virginnaphtha,-suitable for re-forming. is withdrawn as a side stream from atrap-out tray I6 and passes through a line I1 to a cooler I8.

In addition to the naphtha fraction withdrawn from the trap-out tray I6various other fractions may be withdrawn as side streams from the tower8. Thus a virgin gas-oil fraction may be withdrawn from a suitablylocated trap-out tray Il through a line 20.

Regardless of the number or character oi' fractions thus withdrawn asside streams from the tower 8, the residual fractions of the crude arewithdrawn from the bottom of the tower 8 through a line 22 having a pump23, and are delivered to suitable heating means. such as a still or pipecoil 24 conveniently located within the furnace 5. In passing throughthe coil 24 the reduced crude is heated to a temperature suillcient toprovide for vaporization of all but the very heaviest constituentsthereof, and to promote conversion in the presenceof the catalyst withwhich the vaporized portions of the oil are subsequently i brought intocontact. In the instance shown, in which the cracking of this stock iseffected in accordance with the so-called Houdry process, the reducedcrude ordinarily leaves the coil 24 at a temperature of around 880 F. Atthis temperature, the reduced crude passes from the vcoil 24 thmugh avalved transfer line 25 into a separator or vaporizer 26 maintained at alow pressure ranging from atmospheric to about 50 pounds per square inchgauge. Tar bottoms are withdrawn from the vaporizer 26 through a valvedline 21,

wherein is located a cooler 28.

Where it is necessary to provide cooling in the vaporizer 26, a sidestream may be withdrawn near the top of the tower through a line 29leading to a cooler 30 and a pump Il, whence the cooled oil catalyticcontact masses. disposed in any desirable manner, and they are alsoprovided with inlet and outlet manifold connections substantially asshown, in such a manner that they may be alternated as desired.Ordinarily' only one ofthe cases is kept on stream, while the other caseis disconnected for cleaning, regeneration or replacement of thecatalyst contained therein.

While I do not desire to limit myself to any particular catalyst or toany speciilc conditions of temperature or pressure. it may be statedthat a suitable-catalyst comprises an activated hydrosilicate of aluminaand -that typical conditions of temperature and pressure are from 800 to900' 1E'. and from atmospheric to 20 poundsper square inch gaugepressure, respectively, Various details of the Houdry process,applicable to this unit, are disclosed in an article entitled Catalyticprocessing by the Houdry process, found at page R-570 of the NationalPetroleum News for November 30, 1938. and in the patents listed in thatarticle, while various other catalytic processes are described in priorpatents and in the literature.

Under the influence or the catalyst and the heat applied to theoil,conversion takes place, resulting inthe formation of gasoline and otheruseful hydrocarbons. The other vapors leave the on stream catalystcasev34 through a manifold line 36 and pass through the heat exchanger 3into a more or less conventional fractionating column 38, which isoperated to condense and recover intermediate constituents to suchextent as will leave uncondensed a gasoline fraction of the desired endboiling point. The gas-oil condensate'thus obtained is removed from thebottom of the column 30 through a line 31 and where desired may beremoved from the system through a valved branch line 38. The gasolineand lighter vapors and gases pass through aline 39 to a condenser 40 andthence to a separator".

is returned to the top of the vaporizer 28 through a line 32.

The vaporsliberated in the vaporizer 28 pass line I1, after passingthrough the c A portion of the cracked gasoline condensate accumulatingin the separator 4I is conveniently returned to the tower 30 throughaline 43, wherein is located a pump 44, as a refluxing and coolingmedium for the tower 38. The cracked gasoline not required forzreiluxingpurposes in the column 30 is withdrawn from the separator 4I through aline 45.

Uncondensed gases are withdrawn from the separators Il and 4I throughlines 46 and 41. respectively. The subsequent disposition andutilization of these gases will be described in detail hereinbelow, butfor the present it will be sumcient to state that a portion of thesegases ultimately entersanabsorber forming a part of the thermal crackingunit subsequently to be desribed, along with gases` produced in thelatter un The naphtha side stream withdrawn from the column 8 throughthe trap-out tray I 8 and the er I8, is

delivered by means of a pump 5I and line 52 into the upper portion ofthe absorber 50, and' in passing downward through the absorber 50'effects an absorption of the Ca and C4 hydrocarbons, or any desiredportion thereof, from the gases traversing the absorber 50.

The thereby enriched naphtha leaving the absorber $0 and containing C3and C4 hydrocarbons removed by absorption therein. then passes through avalved manifold vapor line 33 into one 75 through a line 52. wherein islocated a pump M,

of a plurality of catalyst cases 34. These catalyst cases are providedinternally with suitable and thence through a heat exchanger Il, a line5l, a heat exchanger 51 and a line I8 into an elongated pipe coil 59 ofrestricted cross-sectional area located within a heating furnace 50. Inpassing through the pipe coil 59, the enriched naphtha is subjected tothermal conversion at an elevated temperature and under superatmosphericpressure. Substantial conversion is obtained at various temperatures andpressures ranging from about 950 to 1400 F. and from 100 to 2000 poundsper square inch gaugepressure, but the best results are obtained whenthe `operation is conducted in the manner set forth and claimed in myprior U. S. Patent No. 2,135,014. That is to say, the admixture ofnaphtha and normally gaseous hydrocarbons is subjected to a highcracking temperature and a high degree of conversion per passsubstantially in excess of the maximum temperature and conversion perpass to which the oll alone could be subjected in identical apparatusand under otherwise identical conditions of conversion without suchexcessive deposition of carbon as to prevent continuous operation of theunit for extended periods of time. The actual conversion temperature,for a given oil, will ordinarily range from 215 to 300 F. higher thanthe aforesaid maximum temperature for the same oil.

During the passage of the naphtha and normally gaseous hydrocarbonsthrough the coil 59, conversion takes place, resulting primarily in theformation of a gasoline product of substantially higher anti-knock valuethan the original charging stock. The heated products are thendischarged through a vapor-transfer line 6 I having a pressure-reducingvalve 62, into a tar separator 64. Under the influence of this pressurereduction and Cooling supplied as will hereinafter be shown, tarryconstituents present in the vapors entering the separator 64 areseparated in liquid form and as such are withdrawn from the bottom ofthe separator 64 through a valved line 65 having a cooler 66. Thetar-free vapors then pass through a line 61, through the heat exchanger51 and thence into a fractionating column 68 of more or lessconventional design.

As the result of cooling supplied to the fractionating column 68,condensation of constituents heavier than are desiredto be retained inthe nal gasoline condensate is effected. Such cooling is readilysupplied by withdrawing a, side stream from the tower 69 through a line69, cooling it in a cooler 10 and returning it by means of a pump 1| anda reflux line 12 to the head of the column 88. It will be obvious,however, that in this and other instances where it is desired to providecooling in one of the units disclosed herein, such cooling may beeffected in any suitable manner, although the apparatus shown in thedrawings represents a simple, convenient and advantageous means.

The condensate recovered in the fractionating column 68, which may bereferred to as gas-oil, is withdrawn from the bottom` of the c'olumn 59through a line 14. All or a portion of this condensate then passesthrough a valved branch line 15, through heat exchangers and 55 to apump 11. Part of the thereby cooled gas-oil is returned by means of aline 18 and a valved branch line 19 to the upper part of the tarseparator 8l to serve as a refluxing and cooling medium therefor. Theremainder of the cooled gas-oil then passes through a line 19 to a pump80 and thence at least in part through a valved quenching line 82 whichcommunicates with the trans- 'I6 fer line Il. The gas-oil thusintroduced into transfer line Il serves as a quenching medium to arrestreactions initiated in the pipe coil 59.

When the stock subjected to thermal cracking consists of naphthawithdrawn from the column 8, comparatively little gas-oil will be formedin the thermal conversion unit. In such instance all of this gas-oilwill ordinarily be returned to the separator 84 or the transfer line 5|or both, and it may be supplemented where desired by the use of cool oilfrom any outside source.

Vapors remaining uncondensed in the column 68 then pass through a line90, wherein may be located a cooler 9 into a condensing and stabilizingcolumn or rectifier 92, wherein removal of stabilized gasolinecondensate is effected. Heat is supplied to the bottom of the column 92by means of heat exchanger 18, which is in communication with the lowerpart of the rectifier 92 through a liquid line 93 and a vapor returnline 94. Stabilized gasoline leaves the system through a cooler 95 and avalved line 96.

Cooling for the top of the rectifier 92 is provided by removing overheadvapors from the top of the column 92 through a line 91 and passing themto a condenser 98 and a separator 99. A portion or all of the cooledcondensate collecting in the separator 99, and comprising mainlycondensed C: and C4 hydrocarbons, is returned by means of a pump |0| anda reflux line |02 to the head of the rectifier 92.

In order to provide for the initial recovery of the C3 and C4hydrocarbons, as well as any higher boiling constituents which may bepresent, from the gases withdrawn from the separators and 4|, all or anydesired portion of these gases is delivered to a compressor |03 (wherethe pressure of these gases is raised to a pressure at least equal tothat maintained in the rectier 92) and then passed into the uppersection of the rectifier 92 through a line |04.

Uncondensed gases leaving the separator 99 pass through a line |05 tothe absorber 50, where C3 and C4 hydrocarbons are removed therefrom byabsorption in the manner set forth hereinabove. Unabsorbed gases (mainlyhydrogen, methane, ethane and ethylene) leave the absorber 50 through avalved gas line |06 and pass out of the system.

In the description set forth hereinabove, I have referred to thecracking or re-forming of straight-run or virgin naphtha recovered inthe initial distillation of the crude. I have not referred to anythermal cracking of any other stock. However, the apparatus shown inFig. l also includes means for effecting cracking of such other stocks,for example, a virgin gas-oil fraction recovered in the initial crudedistillation, an intermediate gas-oil fraction recovered in thecatalytic cracking unit, and also (particularly where the charging stockto the thermal conversion operation comprises oils heavier than naphtha)gas-oil recycle stock produced in the thermal conversion unit itself;all in the presence of -recycled normally gaseous hydrocarbonscontaining 3 to 4 carbon atoms per molecule.

Thus I provide a second pipe coil ||0 located within a furnace forcracking either virgin gas oil or intermediate gas oil produced in thecatalytic cracking operation, or both. Virgin gas oil is delivered tothis coil by means of the line 20, a pump ||2 and a. line I3 while,either alternatively or simultaneously, all or a portion of the gas oilrecovered in the column 36 may be de- -livered to the coil ||0 through avalved branch quenching medium) is deliveredto the coil |30 une mcommunicating wunthe une s1 and wherein is provided a pump ||5. The gasoil thus introduced into the coil is ilrst admixed with a portion of theCa and Ci hydrocarbons recovered in the separator 99, and which may bedelivered for such admixture through a line |20, a pump |21, a line |22and a valved branch line '|23.

The admixture of oil and normally gaseous hvdrocarbons is subjected inthe coil I |0 to thermal conversion under elevated temperature andpressure. The products of conversion then pass through a transfer line|24 having a pressure- .reducing valve |25 into the tar separator 34,

acter of the charging stock delivered to the coil '||0, the temperaturesand pressures employed therein will, of course, be lower than thoseemployed in the naphtha re-forming coil 59.

through a line |32, wherein is located a pump |33. A portion of thenormally gaseous hydrocarbons having 3 to v4: carbon atoms per moleculerecovered in the separator 99 isalso delivered to the coil |30 inadmixture with the gas-oil charging stock by means of the line l' thepump |2|, the line |22 and a, valved branch Y line |34. The temperaturesand pressures employed for conversion in the coil will ordinarily runwithin the ranges setforth hereinabove with respect to the coil ||0,butin each individual case it will usually be found that, be-

i `cause of the refractory character of the recycled Typical operatingtemperatures and pressures will run from 850 to 1000* F.'and from 1000to 2000 pounds per square inch. In any event, however, the best resultsare secured, as in the coil 59, by operating in the manner disclosed andclaimed in my prior Patent No. 2,135,014 and as referred tohereinabove,keeping in mind the character of the individual charging stock.

The entire feed to the coil I|0 will in some cases consist of virgin gasoil withdrawn from the column 3. In other cases the feed will consistentirely of gas oil withdrawn from the column 36, and in still othercases a mixture of both of these gas oils may bedelivered to the coil||0. The optimum temperatures and pressures in each case may of coursevary somewhat, as will be apparent to those skilled in the art, but. arewithin the general ranges set forth hereinabove. Wherever thecharacteristics of the virgin gas oil and the catalytically produced gasoil are so diierent that reasonably optimum results could not be securedby subjecting them to conversion in admixture with each other, separatecoils may be employed for their conversion. Such separate coils are notillustrated in Fig. 1 but would be generally'similar to the coil I0.

In a system of this character, in which the products of conversionentering the vapor separator 64 comprise products of conversion of gasoil as well as products of conversion of naphtha and normally gaseoushydrocarbons, considerable quantities of cracked gas oil may berecovered in the column 68 of the thermal conversion unit. The amount sorecovered will usually be in excess of that required for cooling andquenching purposes. Where this is true, the excess gas oil may either bewithdrawn from the system or it may be recycled for conversion. It ispossible to effect conversion of this stock by recycling it to the coil||0 in admixture with the other oil charged to that coil, but because ofthe refractory character of the cracked gas oil, it is ordinarilypreferable to provide for its conversion in a separate heating coil.

Consequently I have shown in Fig. la heating coil |30 located within aheating furnace |3|.

That portion of the gas oil removed fromthel column 68 (over thatrequired as a cooling and cracked gas oil, somewhat different conditions(either `time or pressure or temperature or any of these) will giveoptimum results in the coil |30; for example somewhat highertemperatures may be employed in thiscoil than in the coil ||0. ProductsVof conversion from the coil |30 are quenched by means of 'cool oilintroduced through a branch line |35 which is in communication throughthe line |26 with the pump 80. The quenched products of conversion thenvpass through a transfer line |36, having a pressure-reducing valve |31,into the tar separator 64.

Distribution of normally gaseous hydrocarbons (in liquefied form) fromthe separator 99 for admixture with the various charging stocks enteringthe coils ||0 and |30 is effected in such manner as to provide 'for aminimum of not less than 15 per cent of normally gaseous hydrocarbons,in terms of the charging stock supplied to each coil on a liquid-liquidvolume basis,

as set forth in my prior Patent No. 2,135,0l4. The amount of normallyhydrocarbon gases delivered to any of these coils may be increased, butordinarily'it will be desirable to hold these down to` the minimumdiluent requirements,v thereby permitting the larger portion of theCsand C4 hydrocarbons to be recycled to the coil 59. 'I'he lattercoilordinarily operates at a higher temperature than any of the otherconversion coils, and is therefore effective to secure a higher degreeof conversion of gaseous hydrocarbons per pass. The 'amount of normallygaseous hydrocarbons (over and above any required for refluxing in thecolumn 92) is readily controlled by regulating the temperature andpressure maintained in the condenser 98 and separator 99. Refrigerationmay be employed if necessary in the condenser 99 in order to secure asulcient volume of liquefied condensate in the separator 99 for suchreuxing and recycling purposes. v

Referring to the system illustratedin Fig. 2, the thermal conversionunit there shown is of the so-called pressure-coking type. The productsof conversion from the naphtha, re-forming coil- 59 pass through atransfer line |50, having Y a valve |5I, into an enlarged coking drumkor chamber |52. No liquid residue is withdrawn from the drum 52, andwhere desired, two or more of these drums may be provided, so that onemay be kept on stream while one or more additional drums arebeingfreedfrom cokev residue separated in the tar separator 34 is removedtherefromthrough a valved line and is delivered by means of a pump |6| and a line|62 into the transfer line |50. Bottoms withdrawn from the vaporizer26iof the catalytic cracking unit are removed therefrom through a line|63 and delivered by means of a pump |54 and the line |62 into thetransfer line |50, along with tar from the tar separator 54. The tarsthus introduced pass (along with the products of conversion from there-forming coil 59) into the coking drum |52, and are there furthercracked and reduced to coke. The latter accumulates in the coking drum|52 and is removed therefrom from time to time.

I have not illustrated in Fig. 2 means for delivering virgin gas oilfrom the tower 8 tothe coil IIII, (although this may be effectedwherever desired). I have, however, shown means for deliveringcatalytically produced gas oil recovered in the column l to the coilIIII.

From the above, it will be apparent that all tars which have beenproposed for use in processes of this character and which, to the extentthat they are individually useful and advantageous in themselves, may beemployed in the catalytic cracking zones of the processes describedhereinabove, are the following: nickel and compounds thereof, such asnickel oxide; chromium and comf pounds thereof, such .as chromic oxide;comproduced ixi the system are eventually'delivered to the coking `drum|52 and there reduced to coke. 'I'he system asa whole yields only coke,gasoline and dry gas as final products. As in the instance described inconnection with Fig. 1, the dry gas -is substantially free of C: and C4hydrocarbons, all the latter (over and above that portion desired to beretained as such in the final gasoline product) being held in the systemuntil converted.

Wherever the heat of the products of conversion from the re-forming coil59 is insufficient to effect the desired amount or degree of coking inthe coking drum |52, the products of conversion from the coil |30 orthecoil M0, or both, may also bedelivered to the coking drum |52 insteadof directly to the tar separator 64. In such instance, no quenchingother than that accomplished by means of the tars introduced into theline |52 (or directly into the coking drum |52) need be provided forwith respect to the products from the coils feeding the coking drum |52.

By virtue of the retention and conversion of Cs and C4 hydrocarbonswithin the system and by virtue of the increased degree of conversionper pass possible with respect to each stock in the thermal catalyticunit, and also by virtue of the application of catalytic cracking tothat type ofstock for which it is best suited and the delivery ofproducts (other than gasoline) from the catalytic cracking unit into thethermal conversion unit for further conversion, the systems describedhereinabove represent extremely advantageous methods of producing highyields of gasoline of high anti-knock value: from petroleum chargingstocks. The catalytic and thermalconversion operations are combined insuch a manner as to secure the optimum advantages of each and-to securecombined effects which could not be secured in either type of operationconsidered alone.

While I have referred hereinabove, primarily by way of example, to theso-called Houdry process, it will be understood that other types ofcatalytic cracking processes, and other catalysts, may be employed inthose instances where catalytic cracking is indicated. Such catalyticcracking processes are numerousand well known in the art and need notIbe catalogued here at length. In general, however, they arecharacterized by the use of low pressures of less than 100 pounds persquare inch, temperatures ranging from "700 to 1100 F., and of coursethe use of a catalyst of one sort or another for promoting thermaldecomposition reactions. Among the catalysts pounds of nickel andchromium, such as nickel chromate; phosphorus compounds, especiallymetaphosphates, including `those of chromium and uranium; aluminas:adsorbent clays; floridin; bauxite; molybdenum sulfide; and a widevariety of other compounds, particularly compounds of metals andalkaline earth metals. Many of these catalysts have entirely differentspecific actions, some favoring hydrogenation, some scission ofcarbon-carbon linkages, some isomerization, some cyclization reactions,and others alkylation reactions. However, for the purposes of thepresent invention, all of these may be considered to come under thesimilar category of catalytic cracking catalysts. And while most ofthese catalysts are preferably employed at low pressures, withregeneration at periodic intervals, my invention (in so far as itdealswith catalytic cracking) is not so limited.

The term normally gaseous hydrocarbons having 3 to 4 carbon atoms permolecule, as used herein, is intended to mean propane, propylene, butaneand butylenes, all of which are normally gaseous in a pure state underatmospheric pressure and temperature conditions. It will be understood,however, that these constituents may or may not exist in gaseous form atdifferent points in the system illustrated. Consequently the termreferred to is not intended to imply that these constituents areactually present as gases, for at many points they will exist in theliquid form by virtue of the pressures employed, or because ofthepresence of liquid oils in which they are absorbed, or both.

As used herein, the term naphtha is employed to designate low-boilingpetroleum fractions Which consist largely' or predominantly ofhydrocarbons boiling within the gasoline boilingpoint range.

While I have described and illustrated my invention hereinabove withrespect to several operating examples and specific operating details, myinvention is not in its broadest aspects limited to such details orexemplications, and may be variously practiced and embodied within thescope of the claims hereinafter made.

What I claim is:

l. A process of manufacturing gasoline of high anti-knock value from apetroleum crude which comprises; distilling said crude to separate itinto a plurality of fractions, including a naphtha fraction and at leastone heavy fraction; subjecting said heavy fraction to conversion in acatalytic cracking zone, and fractionating the products of conversion toseparate a cycle stock fraction heavier than gasoline, a gasolinefraction and residual gases; contacting said naphtha fraction with gasesproduced as set forth hereinbelow to recover hydrocarbons having 3 to 4carbon atoms per molecule by absorption in said naphtha; subjecting thethereby enriched naphtha to conversion in a thermal cracking zone, andfractionating the products of conversion to recover gasoline, a cyclestock fraction heavier than gasoline, and residual gases; coolingcombined residual gases produced in said catalytic cracking zone andsaid thermal cracking zone, under presusure. to recover a liquefiedfraction comprising normally gaseous hydrocarbons having 3 to 4 carbon'atoms per molecule; subjecting gases remaining after separation of saidliqueiled fraction to contact with said naphtha fraction for absorptionas aforesaid; admixing said liquened fraction with at least one of saidrecycle stock fractions, produced as aforesaid, subjecting the ald-.-

mixture of oil and gases thereby obtained to conversion in aseparate'thermal cracking rane, and 'fractionating the crackedproducts-thereby obtained to recover gasoline constituents therefrom.

2. A process as set forth in'claim 1 wherein the cycle stock fractionsubjected to conversion in said separate thermal cracking lione is thecycle stock fraction recovered from the products oi conversion from saidcatalytic cracking none.

3. A process as set forth in claim 1 wherein the products of conversionfrom both of said thermal -cracking zones are combined for fractionationand recovery of gasoline therefrom.

4. 'I'he process as set forth in claim'rwlierein hot products ofconversion from said first-mentioned thermal cracking zone aredelivered. prior to `fractionation thereof, into an enlarged cokingchamber, and residual products separated from said heavier fractionobtained inthe distillation of the petroleum crude prior to the passageof said. heavier fraction through said catalytic cracking zone aredelivered to said enlarged coking zone and there reduced to coke by theheat of the products of conversion from said thermal cracking zone. 5. Aprocess of manufacturing gasoline of high anti-knock value and coke froma petroleum crude, which comprises: dlstilling said crude to separate it-into a plurality ot fractions, including a naphtha fraction and aresidual fraction: vaporizing lighter substances of said residualfraction to separate them from heavier constituents of said residualfraction; subjecting the version to recovera gasoline fraction andresidual gases; contacting said residual gases. under pressure., withsaid naphtha fraction for absorption in said naphtha fraction ofnormally gaseous con- Y stituen'ts having 3 to 4 carbon atoms permolecule: subjecting the thereby enriched naphtha fraction to conversionin a thermal cracking sone: introducing hot products of conversion fromsaid thermal cracking zone and said heavier constituents recovered fromsaid residual fraction into an enlarged coking zone and there reducingsaid heavier fraction to coke; withdrawing products of conversion fromsaid enlarged coking sone: fractionating themto recover heavierconstituents therefrom; and combining residual gases thereby obtainedwith the residual gases from A the catalytic cracking zone. for contactwith said resultant products of conversion are combined ing acne inadmixture with normally gaseous hy drocarbons having 3 to 4 carbon atomsper molefor fractionation with the products of conversion from saidmst-mentioned 'thermal cracking sone'.

'LAprocessasset forthinclaimlhsvhlerein a fraction com g constituentsheavier than gasoline is recove from the products of conversion from thecat lytic cracking sone and subjected to conversion a separate thermalcrackcule. and the products of conversion therefrom are combined forfractionation with the products of conversion from the rstmentionedthermal thereby vaporised constituents of said residual fraction toconversion in a catalytic cracking zone, and fractionating the productsof concraeking lone.

